A high speed turbo engine, such as a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the engine. Due to their complex design, natural resonant mechanical frequencies of the blades may coincide with or be excited by certain blade rotational speeds and rotational harmonics thereof.
To prevent excessive vibration of the blade about its normal position, prudent design practice dictates that the blades be constructed such that the frequencies of the lowest modes fall between harmonics of the operating frequency of the turbine. In addition, the blades may be excited by non-synchronous forces such as aerodynamic buffeting or flutter. To avoid the vibration exceeding certain levels and setting up objectionable stresses in the blades, it is common to monitor the vibrations of the blades, both during the design and testing of the turbine and during normal operation of the turbine.
For example, it is known to use non-contacting proximity sensors or probes to measure blade vibrations. The probes may detect the actual time-of-arrival of each blade as if passes each probe and provide corresponding signals to a blade vibration monitor (BVM). Positional deviations due to vibration are extracted, from which the BVM may determine the amplitude, frequency, and phase of the vibration of each blade. The measured vibration amplitude is highly dependent on correct positioning of the sensor above the blade target, which may comprise a target affixed to the blade, a feature of the blade, or the blade tip itself. In a typical measurement, the probes may be positioned to sense the passage of a leading edge or trailing edge of the blades.
The difficulty in properly aligning a probe is increased by axial movements of the blades during operation, such as may occur as the rotor spins up or down, as the turbine warms up and cools down, as the load on the turbine changes, and due to torsional movements of the blades. At least in view of the foregoing considerations, it is desirable to provide an improved blade vibration measurement apparatus.